More than just a protection layer: Inducing chemical interaction between Li3BO3 and LiNi0·5Mn1·5O4 to achieve stable high-rate cycling cathode materials

Abstract

Spinel LiNi0·5Mn1·5O4 (LNMO), though being considered as a promising cathode material because of their high energy and power density, always suffers from rapid capacity deterioration due to the Jahn-Teller effect and severe leakage of Mn ions into electrolytes at elevated temperatures. In this work, Li3BO3 (LBO) formed from slow hydrolysis of triethyl borate was coated on the surface of LiMn1·5Ni0·5O4 materials prepared through a modified wet-chemistry method. Our results put an emphasis on the strong chemical interactions between the external LBO coating layer and the internal LNMO core induced by a high-temperature thermal treatment; owing to the favorable solid-solid interfacial interaction, the presence of the relatively oxidative Li3BO3 layer was found to diminish the population of the oxygen vacancy and minimize the unstable Mn3+ species in the LNMO active materials, which thus could alleviate the adverse Jahn-Teller distortion and enhance the structural stability. Moreover, as a coating layer, the LBO could prevent direct interaction between the cathode active materials and the electrolyte; as such, the etching of the LNMO cathode materials by the decomposition products of electrolytes such as HF and the leakage of active metal species were inhibited effectively. Besides, the LBO itself is a highly conductive ionic conductor, which thus could enable fast Li+ cation diffusion on the cathode. On this basis, a significant improvement was observed in the high-rate cycling performance for thus coated LNMO cathode materials. The bare LNMO electrode failed within merely 300 cycles. In contrast, our optimal sample LBO2%@LNMO with 2 wt% of LBO delivers a high discharge specific capacity of 127 mAh g−1, which accounts for 92% of its initial capacity after 500 cycles at 1C in the range of 3.5–4.99 V; when cycling at 10 C, it displays an initial discharge specific capacity of 111 mAh g−1 and maintains 85 mAh g−1 after 1000 cycles. Our work demonstrates that in addition to behaving as a physical protection layer, the introduction of the ionic conductive Li3BO3 coating layer could also modify the local composition of the spinel-type LNMO through strong interfacial chemical reaction, which thus could profoundly enhance the structural integrity and high-rate cycling stability.